1. Field of the Invention
The present invention relates to a composition for a plastic magnet widely utilized in various sensors, measuring instruments, motors, automobile parts, electronic parts of an electromagnetic sound generator and the like, and particularly, to a composition for a plastic magnet, containing an Ndxe2x80x94Fexe2x80x94B based alloy powder and a ferrite magnetic material powder as magnetic material powders.
2. Description of the Related Art
There are conventionally known plastic magnets, including a plastic magnet formed from a ferrite magnetic material powder and a resin (referred to as a first example for simplification), a plastic magnet formed from a rare earth metal-based magnet powder (e.g., a samarium-cobalt based alloy magnet and an Ndxe2x80x94Fexe2x80x94B based alloy magnet) and a resin {referred to as a second example for simplification (for example, see Japanese Patent Application Laid-open No. 9-260170)}, and a plastic magnet formed from a ferrite magnetic material powder, a rare earth metal-based magnet powder and a resin {referred to as a third example for simplification (for example, see Japanese Patent Application Laid-open No. 2000-21615)}. Any of the first, second and third examples is formed by any of a compression molding process, an extruding process and an injection molding process. In recent years, an Ndxe2x80x94Fexe2x80x94B based alloy powder having a large magnetic force has been used as the rare earth metal-based magnet powder in many cases.
The first example is inexpensive, as compared with the second example, and has such a preferred characteristic (a plus characteristic) that when it is exposed to a high temperature, the demagnetizing factor thereof is smaller than that of the second example, but has such a minus characteristic that its magnetic force smaller than that of the second example. On the other hand, the second example has such a plus characteristic that its magnetic force is larger than that of the first example, but the second example is expensive, as compared with the first example and has such a minus characteristic that its high-temperature irreversible demagnetizing factor is larger than that of the first example. Therefore, the third example having a characteristic intermediate between those of the first and second examples has been developed. When the first examples are left to stand for 6 minutes in an environment having a temperature of 150xc2x0 C., the high-temperature irreversible demagnetizing factor of the first example is approximately 1%; that of the second example is approximately 6%; and that of the third example is approximately a value intermediate between those of the first and second examples.
In recent years, with automobile parts and electronic parts in which a plastic magnet is used, attempts have been made to reduce the size and weight thereof and moreover, a high quality is required. Especially, with automobile parts and electronic parts used for a long time in an environment having a high temperature, it is required to maintain the quality in the high-temperature environment, namely, it is required that the part has a durability. Therefore, with the third example having the magnetic force larger than that of the first example and capable of being reduced in size and weight more than the first example, it is required that the high-temperature irreversible demagnetizing factor of the third example is further close to that of the first example. However, the plastic magnet conventionally known as the third example merely has the nature intermediate between those of the first and second examples, and demands in the market as described above could not be met sufficiently.
Accordingly, it is an object of the present invention to provide a composition for a plastic magnet, containing an Ndxe2x80x94Fexe2x80x94B based alloy powder, a ferrite magnetic material powder and a resin material, wherein the high-temperature irreversible demagnetizing factor of a plastic magnet formed from the composition can be reduced to smaller than those of the conventionally known plastic magnets.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a composition for a plastic magnet, containing an Ndxe2x80x94Fexe2x80x94B based alloy powder and a ferrite magnetic material powder mixed to a resin material, wherein the Ndxe2x80x94Fexe2x80x94B based alloy powder has particle sizes distributed in a range of 100 to 400 xcexcm; the ferrite magnetic material powder has an average particle size equal to or smaller than {fraction (1/100)} of the particle sizes of particles distributed in a largest amount in the Ndxe2x80x94Fexe2x80x94B based alloy powder; and the weight ratio of the Ndxe2x80x94Fexe2x80x94B based alloy powder to the ferrite magnetic material powder is in a range of 30:70 to 70:30.
With the composition having the first feature, particles of the ferrite magnetic material powder fill gaps between particles of the Ndxe2x80x94Fexe2x80x94B based alloy powder and surround the periphery of each of the particles of the Ndxe2x80x94Fexe2x80x94B based alloy powder. Therefore, the transfer of heat to the particles of the Ndxe2x80x94Fexe2x80x94B based alloy powder can be made difficult by the particles of the ferrite magnetic material powder, and it is possible to exhibit a large magnetic force (intermediate between those of the first and second examples) capable of being distinctly discriminated from that of the first example. Namely, it can be expected that a plastic magnet formed using the composition according to the present invention shows a high-temperature irreversible demagnetizing factor further closer to that of the first example more than to that of the third example, and exhibits a magnetic force equivalent to or larger than that of the conventional third example. Therefore, the plastic magnet formed using the composition according to the present invention can maintain a desired magnetic force over a long period in an environment at a high temperature.
According to a second aspect and feature of the present invention, in addition to the first feature, the ratio of the total weight of the Ndxe2x80x94Fexe2x80x94B based alloy powder and the ferrite magnetic material powder to the weight of the resin material is in a range of 90:10 to 80:20.
With the second feature, it is possible to ensure a sufficient flowability of the composition during the molding thereof, thereby easily molding the composition, and a formed plastic magnet can generate a sufficient magnetic force. Thus, it is possible to sufficiently meet the demands for the reductions in size and weight of the plastic magnet.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.